1. Field of the Invention
The invention relates to a receiver for multivalued digital signals which comprises means for comparing the value of an input signal relative to a reference level with a number of decision thresholds, and generating an output symbol as a result of this comparison. Such receiver includes means for producing a regenerated signal on the basis of the output symbol and the decision thresholds, and means for generating an error signal on the basis of the difference between the absolute value of the input signal relative to the reference level and the absolute value of the regenerated signal. The receiver also includes reference means for deriving the decision thresholds from the error signal.
2. Description of the Related Art
A receiver of this type is known from published European Patent Application No. EP 0 157 598, which corresponds to U.S. Pat. No. 4,841,301, issued Jun. 20, 1989. Generally, such a receiver is used for data transmission by means of, for example, a landline connection or a radio link. To optimally utilize the transmission medium, transmission codes are often used in which the data signal can assume more than two levels. These signal levels usually have equidistant amplitudes and are defined with respect to a reference level which is often equal to 0. An example of such a code is the 2B1Q code in which the logic value of each symbol may assume one of the values +3, +1, -1 or -3. The associated physical signal levels may, for example, be equal to +15 volts, +5 volts, -5 volts and -15 volts.
Because the length of the transmission path and hence the signal loss on the transmission path may vary considerably, the amplitude of the received input signal at the receiver may show considerable variations for different situations. However, the receiver must be able to relate the physical signal value associated with a symbol to the logic value of that symbol despite the considerably varying signal amplitude.
In the prior-art receiver known from said European Patent Application, the decision on the logic value of the output symbol is made by means of a comparison of the value of the input signal to a number of equidistant decision thresholds. In order to minimize the possibility of an erroneous decision, the decision thresholds are to have a value equal to the mean value of two adjacent-amplitude signal levels so that the distance from a decision threshold to each of the two adjacent signal levels is equally large. In the code of said example the optimum values of the decision thresholds are equal to +10 volts, 0 and -10 volts.
Since the amplitude of the input signal at the receiver is not predetermined, the optimum values of the decision thresholds cannot be predetermined either and, therefore, they must be derived from the input signal. This is effected by deriving a regenerated signal from the logic value of the output symbol which is valid at a specific instant and the values of the decision thresholds valid at that instant, which regenerated signal is ideally equal to the physical value of the input signal. The regenerated signal is equal to the product of the logic value of the output symbol valid at a specific instant and half the absolute value of the difference between two decision thresholds valid at that instant.
In order to verify whether the regenerated signal has the correct values, the difference is determined between the physical value of the input signal and the regenerated signal. In order to reduce the difference between the actual and optimum values of the decision thresholds, the absolute values of the decision thresholds are enhanced or reduced respectively, in small steps if the regenerated signal is smaller or larger, respectively, than the physical value of the input signal. Adjustments of the decision thresholds are made in small steps in order to avoid the decision thresholds fluctuating strongly due to occasional disturbances of the input signal as a result of noise, crosstalk and so on.
The method of adapting the decision thresholds in the prior-art receiver poses a problem in that erroneous decisions on the logic value of the received symbol may lead to misadaptations of the decision thresholds, as will be clarified with the aid of the following numerical example. For this purpose, it is assumed that the input signal is a four-value signal having the logic values +3, +1, -1 and -3 and the physical values +15 volts, +5 volts, -5 volts and -15 volts. It is also assumed that the existing decision thresholds in the receiver are equal to -4 volts, 0 volts and +4 volts, which values in an absolute sense are too low because the physical value of the symbol having the logic value +1 is 5 volts. If the transmitter has transmitted a +1 symbol, the input signal of the receiver will be equal to +5 volts. Since the maximum decision threshold in the receiver is equal to 4 volts, the receiver will make the wrong decision that the logic value of the received symbol has the maximum logic value of +3. The corresponding regenerated signal will therefore be equal to +6 volts (+3*4 volts/2). As the absolute value (6) of the regenerated signal now exceeds the absolute value(s) of the input signal, it will appear to the receiver that the absolute value of the decision thresholds is too high and so the absolute decision threshold values will be lowered, although they are actually already too low. A similar misadaptation, to increase the absolute value of the decision thresholds, may also occur when they are already too high.
The misadaptations of the decision thresholds may lead to a constant or lengthy erroneous symbol detection as a result of which the receiver will not operate properly or only after a lengthy adjusting time.